Studies were undertaken to determine the prevalence and natural behavior of type C RNA tumor viruses of the cat and the endogenous retrovirus of guinea pigs. Preliminary sero-epidemiologic survey of cats in India suggested that the incidence of feline leukemia virus infection in these cats is low or undetectable at the level surveyed. A sensitive radioimmunoassay (RIA) for the major structural proteins of the chemically inducible guinea pig endogenous retrovirus (GPV) suggested that GPV expression in guinea pig cells and tissues is not correlated with neoplastic transformation and that GPV is not related to the intracisternal type A particles observed in transplantable guinea pig leukemias. Previously described interspecies immunoassays indicating that distantly related retroviral proteins contain common antigenic determinants has promoted the development of a search and use of monoclonal antibodies capable of recognizing these determinants.